Sulfuric acid refining of benzene



Dec. 2, 1958 S. NORD SULFURIC ACID REFINING OF BENZENE Filed May 17,1957 United States Patent O SULFURIC ACID REFINING F BENZENE Sol Nord,East Orange, N. `J.

Application May 17, 1957, Serial No. 659,829 3 Claims. (ci. 26o-674)This invention relates to the refining of benzene, i. e. to theproduction of high purity benzene from mixtures of benzene with toluene,olens, sulfur compounds, and other impurities. More specifically, itrelates to a process of producing high purity, high freeze-point, lowbromine number, low sulfur-content benzene from coke oven light oil,coke oven light oil distillates, petroleum fractions, or other mixturescontaining bezene, olens, and sulfur compounds such as carbon disuldeand/or tiophene.

Prior art In the rening of benzene-containing mixtures such as coke ovenlight oil or coke oven light oil distillates, it is conventionalpractice to treat the mixture with concentrated sulfuric acid beforedistillation. The sulfuric acid reacts with the unsaturated compoundspresent in the mixture, forming a sludge which settles out with thespent acid. The result is a decrease of unsaturation in the nal benzeneproduct, as indicated by its acid wash test and bromine number. It alsomakes it possible to obtain freeze-points in the nal benzene productwhich approach the freezing point of pure benzene (5.4 C.) more or lessclosely. It is also known that unsaturated sulfur compounds such asthiophene are partially removed by this treatment, and that the sulfurcontent of the final benzene product is accordingly also decreased.

In certain cases, it is difficult to obtain sufficient puriiication bythis treatment, e. g. it may be difhcult to meet specifications of acidwash test, bromine number, freezepoint, or sulfur content. In general,all that can be done Aabout this is to increase the amount of sulfuricacid used in the acid washing treatment. This, however,

does not always solve theproblem, and is expensive in Furthermore, nounusual or complicated equipment is required. Conventional acid washingand distillation equipment are used.

Objects An object of my invention is to provide a process for theproduction of benzene of improved purity from benzene-containingmixtures.

A second object of my invention is to provide a process for theproduction of benzene of improved purity from benzene-containingmixtures, without the use of novel equipment or chemicals, and with verylittle increase in sulfuric acid consumption or decrease in final yieldof product.

A third object of my invention is to provide a process for improving theacid wash test, sulfur content, bromine number, and freeze-point ofbenzene over that obtained by conventional processes, without the use ofnovel equip- 2,862,983 Patented Dec. V2, 1958 ICC v Advantages Anadvantage of my invention is that no novel or expensive equipment isrequired.

A second advantage of my invention is that no novel or expensivechemicals are required.

A third advantage of my invention is that the increase in acidconsumption is very small.

A fourth advantage of my invention is that there is substantially nodecrease in final yield of product.

In the accompanying figure, I have shown a schematic process flow-sheetof a preferred embodiment of my invention. For the purposes ofillustration, I have used coke oven light oil as the material to betreated, but I may use other benzene-containing matereals instead.

Description The starting point of my process is a conventional acidwasher 10, equipped with suitable means for agitation 11, which may forexample consist of a mechanically driven agitator or a recirculatingpump. Concentrated sulfuric acid, preferably 66 B. HZSO.,= is added at12 to a mixture of coke oven light oil 13 and recirculated B-Tdistillate, the source and composition of which is described below.Spent acid and acid sludge are withdrawn at 14, and the acid-washedlight oil is withdrawn at 15. This acid-washing step can be performedbatchwise or continuously, and as it is well known in the art, is notdescribed further here.

The acid-Washed light oil is then pumped by pump 16 to a neutralizerr17, which contains suitable means for agitation 18, of a type similar tothat used in the acid washer 10. Dilute NaOH solution is added at 19, inorder to neutralize acid entrained in the oil. Spent alkali is withdrawnat 20, and neutralized oil at- 21. VThis neutralization step can also beperformed batchwise or continuously, and as it is well known in the art,is not described further here.

The acid-washed, neutralized oil is pumped by pump 22 to an intermediatesection of a forerunner fractionating tower 23, of a conventional type.Benzene forerunner vapors are taken overhead at 24 and condensed incondenser 25. A portion of the condensate is returned as reflux at 26,and the remainder is withdrawn as benzene forcrunners at 27. Bottoms arewithdrawn at 28, dropping into a conventional reboiler 29. Vapors 30 arereturned to the bottom of the -tower 23, and forerunnerfree bottoms arewithdrawn at 31. In the operation of the forerunner column 23, asufficient fraction is taken overhead, with suicient fractionation, sothat the bottoms are substantially free of benzene forerunners, i. e. ofmaterial boiling below benzene. Since this operation is conventional, itis not described further here.

The forerunner-free bottoms from fractionating tower 23 are pumped bypump 32 to an intermediate section of a benzene fractionating tower 33of a conventional type. Pure benzene product vapors are taken overheadat 34 and condensed in a condenser 35. A portion of the condensate -isreturned as reflux 36, and the remainder is withdrawn as pure benzeneproduct at 37. Bottoms are withdrawn at 38, dropping into a conventionalreboiler 39. Vapors 40 are returned to the bottom of tower 33, andbenzene bottoms are withdrawn at 41. The operation of this tower 33 isconducted in a novel manner, and constitutes a critical part of myinvention. Instead of taking all -the benzene voverhead as product, apredeterfeed tothe benzene tower 3-,and the amount of those impuritiesboiling in .the benzene range which normally cause high wash tests,sulfur content, and bromine numbers, and low freeze-points in thebenzene. This optimum concentration should thereforebe determined byexperi- .ment in any given case.

The benzene bottomsfrom the benzene tower 33 are `now pumped by pump 42to an intermediate section of a B-T fractionating tower '43 of aconventional type.

B-T (i. e. a mixture consisting of benzene, benzene impurities, andtoluene) vapors are taken overhead at 44 and condensed in a condenserAS.A portion of the condensate is returned as refluxA, and the remainder iswithdrawn as B-T distillate at 47. Benzene-free bottoms are withdrawn at48, dropping into a conventional reboiler49. Vapors 50 are returned tothe bottom of the tower 43, and benzene-free bottoms are withdrawn at51, The operation of this tower is such that no substantial amount ofbenzene remains in the bottoms, and that substantially all the benzeneand benzene impurities are taken overhead, withas little toluene aspossible. Any toluene going overhead beyond the amount necessary toinsure that all the benzene and impurities are taken overhead, .isunnecessarily recycled. Thus, the toluene concentration of the overheadwill depend largely on .the fractionating eflciency of the tower 43. Thebenzene-free bottoms are lthen available for working up into puretoluene and heavier fractions by conventional means. The B-T distillate,on the other hand, is pumped by pump 52 back to the washer 10 forre-washing, along with a fresh batchof coke oven light oil (or othercharge material).

I have found thatif thefbenzene contained in the B.*T fraction soproduced isdirectly recovered by fractional distillation in theconventional way, without re-washing,

tent, and bromine number, and a low-freezepoint. For example, thisbenzenemay have a bromine number over 500, and often over 1500. It mayhave a sulfur contient of 0.1%, and often as high as 0.3%. The acid washtest is -generally ,worse than number 6, and the freeze point isgenerally down to 2-3 C. These results are contrary to what one wouldexpect, since there is no reason to suppose, `nor any literature orplant experience to indicate that the benzene impurities which causethese results can be :fractionated out of benzene. On the contrary theygenerally require doubling ortripling the amount of sulfuric acid usedin the washing, vsimply because they cannot be removed in any otherknown way.' Alternatively, very expensive types of benzene purificationplants have been proposed, including solvent extraction and otherexpensive operations. However, I have found that these impuritiesconcentrate in the intermediate B--T fraction, produced by; the methoddescribed above. In a sense, they seem to be entrained with the tolueneand heavier bottoms from tower33, as if the toluene and heavier werefsolvents in anextractive distillation process. Whatever the reason maybe, we have found that4 the benzeneimpurities can be more or lessisolated in the -Bf-T distillate fraction so produced, and thus kept outof both thepbenzene and the toluene fractions. I have also found thatrc-washing this B--T distillate fraction, along with the regular chargematerial, eliminatesthese impurities with virtually a neglligibleincreasein acid-consumption or decrease in yie d.

Thus, it will be seen that I have provided an extremely .such benzenehas unusually high wash test, sulfur consimple and inexpensive method ofimproving the purity of benzene, i. e. 4wash test, sulfur content,bromine number, and freeze-point, without the use of any novel equipmentor chemicals.

Other modifications in the details of operation may also be made withoutdeparting from the spirit of my nvention, the principal novel featuresof which are set forth below.

I claim:

l. In a process for producing benzene of high purity, in which abenzene-containing charge mixture is contacted with concentratedsulfuric acid to react with impurities, treated with alkali toneutralize the remaining acid, and then subjected to fractionaldistillation to produce pure benzene and other products, the novel stepscomprising: producing by fractional distillation an overhead fractioncomprising primarily benzene and being substantially free of `heaviercomponents, and a residual comprising substantially all the componentsheavier than benzene together with a predetermined quantity of benzenewhich is sufficient to contain the major portion of the remainingimpurities boiling in the benzene range; producing by further fractionaldistillation of said residual an intermediate overhead fractioncontaining substantially all the remaining material .boiling in the.benzene range ytogether with some higher-boiling material, and aresidualsubstantially comprising components heavier than benzene; andrecycling said intermediate overhead fraction for further contactingwith sulfuric acid and re-fractionation.

2. In a process for producing b enzeneof high purity, in `which abenzene-containing charge mixture is contacted with concentratedsulfuric acid to react with impurities, treated with alkali `toneutralize the remaining acid, and then subjected to continuousfractional distillation in a multiplicity of fractionating towers toproduce pure benzene and other products, the novel steps comprising:making a cut between acidfalkali-treated Abenzene and heavier componentsin which the `overhead fraction is substantially free of heavierAcomponents,.and thebottoms fraction contains a predeterminedconcentration of benzene which is suicient to contain the major portionof the remaining impurities vboiling in the benzene range; subjectingsaid bottoms to further fractional distillation to producean overheadcontaining substantially all the remaining material boiling in thebenzene range together with somehigher-boiling material, and a bottomfraction which is substantially free of benzene; and recycling saidoverhead containing benzene and higher boiling material for furthercontacting with sulfuric acid, and re-fractionav tion.

3. In a process for producing benzene of high purity, in which abenzene-containing charge mixture is contacted with concentratedsulfuric acid to react with impurities, 4treated with alkali toneutralize the remaining acid, and then subjected to continuousfractional distillation in a multiplicity of fr actionating towers toproduce pure benzene and other products, the novel steps comprising:subjectingcharge material which has been contacted with acid and alkalito continuous fractionaldistillation to produce an overhead fractioncomprising substantially all the benzene forerunners, and a firstbottoms fraction which is ,substantially free ofmaterial boiling belowbenzene; subjecting said first bottoms fraction to a second continuousfractional distillation to produce an overhead fraction comprisingsubstantially pure benzene product, and a second bottoms fractioncomprising heaviercomponents together with a predetermined quantity of.benzene which is sufficient to contain the major portion of theremaining impurities boiling in the benzene range; subjecting saidsecond bottoms fraction to a thirdcontinuous fractional distillation toproduce a recycle overhead fraction containing substantially all theremaining material boiling `in the benzene range `together with somehigher-boiling material, and a substantially benzene-free bottoms frac-5 tion; returning said recycle overhead fraction for further acidcontacting, along with fresh charge material, and subsequentrefractionation; land subjecting said benzenefree bottoms fraction tofurther fractional distillation t0 produce additional productstherefrom.

References Cited in the ile of this patent UNITED STATES PATENTS

1. IN A PROCESS FOR PRODUCING BENZENE OF HIGH PURITY, IN WHICH ABENZENE-CONTAINING CHARGE MIXTURE IS CONTACTED WITH CONCENTRATEDSULFURIC ACID TO REACT WITH IMPURITIES, TREATED WITH ALKALI TONEUTRALIZE THE REMAINING ACID, AND THEN SUBJECTED TO FRACTIONALDISTILLATION TO PRODUCE PURE BENZENE AND OTHER PRODUCTS, THE NOVEL STEPSCOMPRISING: PRODUCING BY FRACTIONAL DISTILLATION AN OVERHEAD FRACTIONCOMPRISING PRIMARILY BENZENE AND BEING SUBSTANTIALLY FREE OF HEAVIERCOMPONENTS, AND A RESIDUAL COMPRISING SUBSTANTIALLY ALL THE COMPONENTSHEAVIER THAN BENZENE TOGETHER WITH A PREDETERMINED QUANTITY OF BENZENEWHICH IS SUFFICIENT TO CONTAIN THE MAJOR PORTION OF THE REMAININGIMPURITIES BOILING IN THE BENZENE RANGE; PRODUCING BY FURTHER FRACTIONALDISTILLATION OF SAID RESIDUAL AN INTERMEDIATE OVERHEAD FRACTIONCONTAINING SUBSTANTIALLY ALL THE REMAINING MATERIAL BOILING IN THEBENZENE RANGE TOGETHER WITH SOME HIGHER-BOILING MATERIAL, AND A RESIDUALSUBSTANTIALLY COMPRISING COMPONENTS HEAVIER THAN BENZENE; AND RECYCLINGSAID INTERMEDIATE OVERHEAD FRACTION FOR FURTHER CONTACTING WITH SULFURICACID AND RE-FRACTIONATION.